1. Field of the Invention
The present invention relates to a novel optical switch making use of the excitation phenomenon of a nonlinear optical thin-film caused by a control light to rapidly switch optical paths for a signal light containing electronic information and to perform switching of an optical signal, an element therefor, and an optical switching method.
2. Related Art    Non-patent document 1: Nikkei Electronics, Aug. 13, 2001    Patent document 1: JP-B-3-24650    Patent document 2: JP-A-8-82811    Patent document 3: JP-A-2000-314901
With the development of an information-oriented society, a large amount of electronic information is communicated on optical fiber networks, and fast response is demanded for switching elements, which serve as “point of intersection” for disposing of optical signals between networks as interconnected. Conventionally, switching elements of a optical-electrical-optical type are general to once convert an optical signal into an electric signal to convert the same again into an optical signal to send the same to a network being a destination of transmission. Since parts cost and power consumption required for photoelectric conversion are increased with an increase in information throughput, however, attention has been given to optical switches, which dispenses with conversion into electric signals and can perform routing switching of an optical signals directly. With such optical switches, an optical mechanism performs switching not to be dependent upon data transmission rate, and so it is easy to deal with transmission rate of at least 10 G bit/second, with which an electric switch is difficult to deal.
Optical switches include a mechanical type MEMS (Micro Electro Mechanical Switch) making use of a micromirror and an electrostatic actuator, and planar waveguide type switch, which are described in Non-patent document 1. Switches have been developed to have a switching time in the order of milliseconds.
Meanwhile, switches making use of an optical effect to perform a very high speed switching include an optical switch described in Patent document 1, in which a waveguide is provided on an electrooptic crystal made of lithium niobate, or the like, and switching is performed by a change in refractive index, caused upon application of an electric field. With such optical switch, application of an electric field causes a change in the refractive index of the electrooptic crystal and adjusts the refractive indexes of surrounding materials so that a signal light incident obliquely upon the electrooptic crystal is put on a condition of total reflection and on a condition of non-total reflection, whereby optical paths are switched. Also, optical switches are known from Patent documents 2 and 3, which perform a very high speed switching in the order of femtoseconds owing to the nonlinear optical effect caused not by an electric signal but by a control light.
In order to materialize optical switches enabling a very high speed switching, it is favorable to make use of a refractive index change caused by the nonlinear optical effect resulted from electronic excitation, which exhibits fast response in the order of picoseconds. Since an amount of refractive index change caused by the nonlinear optical effect increases with an increase in energy density of a control light 4 (exciting light), it is necessary to increase the energy density of the control light 4 in order to obtain an adequate and stable switching action. As measures for this, measures for increasing a light source output of a control light, and measures for converging the control light 4 are conceivable.
When the former measures are adopted, an absolute energy amount being input is increased to weaken the energy saving quality of an element. Further, when a part of the energy is converted into heat in the course of energy relaxation after excitation to be reserved and radiational cooling cannot accommodate such heat reserve, there is a possibility that an element is raised in temperature. Since temperature rise also changes an optical characteristics of an nonlinear optical thin-film, there is a fear of weakening stability of a switching action.
In this manner, the latter measures for converging the control light 4 is favorable in view of energy saving quality and stability in performance. In the case of pulse excitation in the order of nanoseconds and picoseconds, an energy density in the range of GW per square meter is necessary in order to obtain an adequate nonlinear optical effect. In the case of using laser having an output in a convenient range of mW as a source of the control light 4, the light is converged to a size of μm to thereby materialize the energy density. In examining refractive index changes of a metallic oxide thin film, which exhibits an nonlinear optical characteristics, in a wavelength region of a signal light 3, the changes being caused by the pulse control light 4 of with femtoseconds laser, attenuation changes in the order of nanoseconds were observed in addition to response in the order of picoseconds, which was considered to be resulted from electronic excitation/relaxation phenomenon.
The latter attenuation changes in the order of nanoseconds are considered to be a phenomenon due to thermal relaxation, and in connection with this phenomenon, heat transfer affects surroundings of a portion being irradiated. Meanwhile, since a region, in which a refractive index changes in large amount and the former electronic excitation/relaxation takes effect, is restricted to a local spot irradiated by the control light 4, it has proved from experiments that a signal switching efficiency is sharply lowered when overlapping of a spot of the control light 4 and a spot of the signal light 3 is inadequate. That is, the signal light 3 getting out of the spot of the control light 4 behaves in the same manner as before excitation to be taken out as a background irrespective of presence and absence of excitation, so that switching is decreased in contrast ratio.